Cardiac Action Potential Phases

Cardiac Action Potential Phases is the sequence of distinct voltage changes that a cardiac muscle cell undergoes during a single cycle of excitation, conventionally divided into five numbered phases, each corresponding to a characteristic pattern of ionic current flow across the cell membrane that together produce the prolonged depolarization distinctive of cardiac tissue.

Phase 0: Rapid Depolarization

Sodium Current Upstroke

Phase 0 is characterized by a rapid, large-amplitude depolarization driven by the sudden opening of voltage-gated fast sodium channels, producing a large inward sodium current that drives the membrane potential rapidly from its negative resting value toward a positive value.

Phase 1: Early Repolarization

Transient Outward Current

Immediately following the upstroke, a brief, partial repolarization occurs due to inactivation of the fast sodium channels combined with activation of a transient outward potassium current, producing a small notch in the action potential waveform before the plateau phase begins.

Phase 2: The Plateau

Balanced Inward and Outward Currents

Phase 2, the plateau, is the hallmark of the cardiac action potential, sustained by a balance between inward calcium current, entering through voltage-gated L-type calcium channels, and outward potassium current, which together hold the membrane potential relatively stable near zero millivolts for an extended duration.

Functional Significance of the Plateau

The prolonged plateau extends the duration of the action potential to nearly match the duration of mechanical contraction, ensuring an extended refractory period that prevents premature re-excitation and the sustained, fused contraction known as tetanus.

Phase 3: Rapid Repolarization

Dominance of Outward Potassium Current

As the inward calcium current gradually inactivates, outward potassium currents, including delayed rectifier channels, become dominant, driving the membrane potential rapidly back toward its negative resting value and completing repolarization.

Phase 4: Resting Potential or Pacemaker Potential

Stable Baseline in Working Myocardium

In working atrial and ventricular myocardial cells, phase 4 corresponds to the stable resting membrane potential, maintained near the potassium equilibrium potential until the next wave of excitation arrives from a neighboring cell.

Spontaneous Depolarization in Pacemaker Tissue

In cells of the sinoatrial and atrioventricular nodes, phase 4 instead consists of a gradual, spontaneous depolarization, driven by a combination of declining potassium conductance and activation of specific inward currents, that carries the membrane potential toward threshold and initiates the next action potential without external stimulation.

Regional Variation Among Phases

Differences Between Nodal and Working Tissue

Pacemaker cells of the sinoatrial and atrioventricular nodes exhibit a modified action potential lacking a distinct phase 1 and a rapid phase 0, instead relying on a slower calcium-mediated upstroke, reflecting the different ionic channel composition of nodal tissue compared to atrial or ventricular myocardium.

Consistency of the Plateau Phase

Despite regional variation in upstroke and pacemaker properties, the sustained plateau phase, dependent on calcium influx, remains a shared feature across most cardiac tissue types, underlying the general refractory characteristics of the heart as a whole.

Integration with Mechanical and Electrical Function

The sequential progression through these phases, from rapid depolarization through the sustained plateau to repolarization, establishes both the electrical signal that triggers calcium release for contraction and the extended refractory period that shapes the rhythmic, non-tetanic pattern of cardiac muscle activity essential to effective pumping function.